If this sentence is changed to passive voice, then the subject is also changed, in order to retain the meaning of the sentence.

So, the object takes the place of the subject and vice versa and the word 'by' is introduced as now the subject receives the action by the object. The verb form of the sentence is also changed from active form to passive.

'Subject (Mohan) + verb (gave) + indirect object (the beggar) + direct object (an old shirt)' changes to:

'New subject (An old shirt) + verb (was given) + direct object (to the beggar) + by + new object (Mohan)'

The passive form of 'gave' in the simple past tense is 'was/were given'.

This can be seen in option D, hence it's the right answer.

Option A: The indirect object 'the beggar' and subject 'Mohan' have changed places, such that the beggar is performing the action. But, the beggar is receiving the action in the given sentence 'Mohan gave the beggar'.

So, this option does not have the correct passive form.

Option B: The passive form of 'gave' is 'was given', as opposed to the option here 'was gave'.

Option C: Similarly, the verb form here 'was gave' is not the correct form.